Tubes of the type having larger diameters at the base and smaller diameters at the top, are often used for street lights, tall signs, etc. Many times, these are made from nested cylindrical tubes of varying diameters which nest within one another and are welded together at their junctions. These techniques are generally slow and expensive and require more metal than necessary for the proper strength. These operations require machinery which is both costly and space consuming.
Alternatively, tubular members are tapered to a variety of useful shapes by conventional swaging operations. In these operations, the uniform wall thickness of the tubular member is swaged to reduce the diameter of the member at selected portions with the consequent result that the wall thickness increases at the reduced diameter portion. The swaging process includes multiple steps which are different and separate from each other. An initial stock must be passed through the step-by-step procedures until it can finally be finished to a desired shape. The steps start with cutting a flat metal blank to a tapered shape, followed by forming it to a tapered tubing with the opposite longitudinal edges left unjoined together. Then, the next step causes the edges of the tubing to be joined together by means of a welding process. Any undesired portions that are present along the welded edges of the tubing are removed by means of a polishing or finishing process. The manufacture of such a product involves many steps, each including different processes. As a whole, a combination of these processes permits the manufacture of the product. However, it is a highly inefficient mass production technique. This swaging process also increases the manufacturing cost for each item.
Single-step swaging processes are also known, in which the swaging machine is specifically designed to provide a small-length tapered metal tubing (such as forks for bicycles). For long tapered tubes which are between 12 and 30 feet long, such as lighting standards, it is practically impossible to implement a single-step swaging process.
Another type of tube tapering machine has been devised wherein tubular metal stock is rotated about its longitudinal central axis. During this rotation, spinning rolls or shoes work on the exterior of the tube to deform it from its initial cylindrical shape to a smaller diameter of either straight or tapered shape. The surface presented against the work piece or tubular stock by the rolls or shoes tends to bend or flex the metal of the tube wall in a direction opposed to its circumferential curve or configuration. This flexing, which is repeated revolution after revolution of the tube, has a tendency to produce longitudinal cracks or to propagate insignificant scratches on the tube wall into cracks. This is particularly true on the inner surface of the tube.
U.S. Pat. No. 3,879,977, issued on Apr. 29, 1975, to F. B. Cauffiel, describes an apparatus and method for producing tapered poles. In this patent, the tapered pole is made from a particular tapered strip which is wound in a pseudo-helical manner on a tapered mandrel. Suitable means are provided for directing and guiding the strip on the mandrel such that the strip is wound from the small end of the mandrel toward the large end. Welding procedures are provided so that the edges of the strips can be formed together so as to produce a tapered pole.
U.S. Pat. No. 3,735,463, issued on May 29, 1973, to A. Merola describes a method of forming tapered tubular members. In this method, a generally uniform wall thickness, hollow, cylindrical body is utilized as the starting material. The body is elongated by engaging the body at two spaced portions along its length. By applying a predetermined force to the body of the engagement portion, a substantial component of the force is directed parallel to the longitudinal axis of the body. When the body elongates between the engaged positions, the wall thickness of the elongated portion is reduced a predetermined amount. The process described in U.S. Pat. No. 3,735,463 was designed for the production of ball bats through a conventional type of swaging process.
U.S. Pat. No. 3,041,990, issued on Jul. 3, 1962, to C. K. Le Fiell describes a tube tapering machine. This machine tapers tubes by using a tool which works on the metal of the workpiece to deform it. This tool is in the form of an annular ring presenting a concave surface or line of contact toward the convex surface of the tubular stock.
U.S. Pat. No. 4,622,841, issued on Nov. 18, 1986, to K. Yoshida shows a single-pass swaging operation. In this invention, several swaging units are arranged in series in a tandem configuration. Each swaging unit includes a flywheel which acts as an anvil. It also includes a set of metal dies. In the tandem configuration, the die sets and the swaging units are arranged sequentially from one to another. On the entry side of the tandem configuration machine, a tubing stock of a prescribed length (which is to be tapered) is inserted, progressing through the machine toward the other side thereof. While the tubing stock is being fed forward, it is sequentially processed by the dies in the swaging unit that provide the tapered shape over the length. After having been processed through all of the dies, the stock is formed to a totally tapered shape.
U.S. Pat. No. 3,330,145, issued on Jul. 11, 1967, to G. F. Adolphi describes a machine and method for tapering rod-like tubular workpieces. In particular, this patent describes the technique in which a cylindrical tube is gripped at both ends and a tension is applied axially to the tube. The tube does not rotate during the forming pass but is rotated at the completion of each pass. This device is generally intended for the manufacturing of tapered tubes having a diameter of less than one inch.
In this Adolphi patent, the die orifice is round on the section through the axis of die segment rotation. At all other sections, it consists of concentric circular arcs with gaps therebetween. The orifice, so formed, is diametrically smaller adjacent to the gaps than at the root of the groove. In this configuration, the workpiece exiting the die is not perfectly round. This causes a limitation in the allowable workpiece reduction. A reverse curvature or concavity is created at the die segment juncture which causes the tube wall to buckle inwardly when subjected to the circumferential compressive force of the drawing operations.
It has been generally found that the Adolphi process may work properly on tubes having relatively small diameters (less than one inch). However, problems can be created using the Adolphi process when used in conjunction with the tapering of tubes having a greater diameter than one inch. It is important to be able t o achieve reductions of between fifteen to twenty-five percent on the diameter of the tapered tube. The reductions in diameters which were achieved by the Adolphi patent would be between 0.010 inch to 0.030 inch on tube diameters of between 0.3 inches and 0.7 inches.
It is an object of the present invention to provide an apparatus and method for tapering tubes that reduces the requirements of die pressure
It is another object of the present invention to provide a tube tapering method and apparatus that prevents collapse of the tube wall during the forming process.
It is still another object of the present invention to provide a tube tapering apparatus and method that provides a tapered tube having generally constant wall thickness.
It is still a further object of the present invention to provide a tube tapering apparatus that minimizes space requirements.
It is still another object of the present invention to provide a tube tapering apparatus and method that enables tapered poles to be manufactured at minimal manpower and material cost.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.